1. Field of the Invention
This invention relates to a frequency shift keying system, and more particularly to a frequency shift keying system for providing modulated output signals of frequencies which differ from the input level of an input code.
2. Description of the Prior Art
There are several existing modulation systems, for example, a system which changes the inductance and the capacitance of an L-C oscillator in accordance with the level of an input code, a system employing an astable multivibrator, and so forth.
However, these conventional systems have the following defects as: The kind of system which switches the inductance and the capacitance of the L-C oscillator is defective in that a fractional bandwidth of the output frequency cannot be made large, that when reactance is changed, a phase shift occurs in the output signal resulting in increased modulation distortion, and thus a circuit adjustment is required. The system employing the astable multivibrator has the drawbacks that the circuit construction is complicated and that frequency stability is low, that is, the output frequency is liable to vary with temperature or input code voltage fluctuation.